Water-repellent aerogels



United States Patent 2,805,961 WATER'REPELLENT AEROGELS,

iraiE. Puddington and Aurelio F. Sirianni, Ottawa, ntario, Canada, assignors to National Research Council, Ottawa, Ontario, Canada, a body corporate of Canada No Drawing. Application March 5, 1952, Serial No. 275,016

10 Claims. (Cl. 117-100) ingan aspect of rigidity and with the appearance, even microscopically, of homogeneity. The gel was subsentl r The customary dried. gel used in commerce wasfthe silica gel, formed by the interaction of a solution of Water glass with, an acid, the combination resulting in the formation, after standing, of'a rigid gel. brokenup, thoroughly washed to remove salts and'dried, the gel shrunk considerably. When completely dry it presented a hard glassy mass which was. characterized y h e t a u s ue ns nd ry n n he same menstruum did not afiect in any substantial man-. ner thephysical characteristics of'the originalv product. Thevaluable feature of the dried gel lay, of;course, in the production ot a mass which possessed up to 50% voids by volume and which, in some instances and depending on various treatments in themanufgcture there of, possessed even higher voids. li a. and other ss r hank zab n omai s.- vented largely the use and advantages of such dried materials with their enormous internal surfaces.

A study of the drying operation suggested theoretically that the gel forr'nedwasfamass. of: interlacing fibres, that on removal of the liquid menstruurn by drying the liquid withdrew into and betweenlhe mass of submicroscopic fibres, where, aided by capillary attraction, its surface exerted tension upon the minute fibrils so that not only a loss of the water alone-tookplace-but a shrinkage of the entire mass. 7

It was apparent, therefore, that if the above drying process could be controlled, or in some Way circumvented altogether it might be possible to produce a'dried" gel which would yield an entirely new class of colloidal products of an essentially amorphous characterin which the voids between the minute individual particles or fibres were substantially of the. same'character and size as within the undried gel in its original state.

By the aforementioned disclosure, U. S. Patent No,

2,093,454, and-in subsequent U. S. patents numbered 2,188,007 and 2,249,767, Kistler madeknown-a method whereby he produced gelswith void-spaeeranging-up to 99% by volume and even higher. To distinguish between the gels heretofore described he namedthese aerogels.

However, the aerogels produced in the past,- suchas those described inthe afore-mentioned Kistlenp'at'ents,

ein

Bu the. e en 2,805,961 Patented Sept. 10, 1957 suffer from the physical characteristic that they are affected by water and humidity with the undesirable featurc'that they subsequently shrink on drying out-either partially or completely depending on the amount of liquid absorbed or adsorbed--to leave an amorphous product of less value in commerce than that of the original dried aerogel. Further, the use of such aerogelsas carriers for catalysts are seriously affected in these; types of reactions, particularly in gaseous chemical reactions wherecondensed water is present; thus tending to balance out and nullify the original economic advantage oithe aerogel. Also, these types of aerogels cannot be-usedsatisfactorily as a final and beneficial product in those instances: where: water is met with or may be metwith' in a liquid" or vaporous form, such as fillers in life preservers, smoke" and gas masks employed in fighting fires, heat insulating material in exposed conditions, ,etc. Further these prod;- ucts do not provide the most, favourable results; when used as oilthickeners in greases and other lubricants.

It is a primary object of the present invention, there-' fore, to produce an aerogel by. a method over whichcomplete control may be exercised at all phases yet' at, the same time providing a product having water-repellent characteristics.

It is a further object to produce an aerogel which, when water is added'thereto, does not substantially shrink" on drying.

It is, an additional object to provide a product which. in itself is of commercial'value,.with anextremely high volume to weight ratio, and which is substantially impervious to humidity.

It is a further object to provide awater-repellent aerogel in an economical and simplejmanner,

it is a still further. object of this invention to provide a water-repellent aerogeljwhich may be used as anad'di tive product in the manufacture fjgreaSeS, Paints anticaking agents, printing inks, varnishes, lacquers, rubber and the like.

Without limiting the objects. aforesaid in any way,-, the manufacture of a typical water-repellent aerogel which may be used to illustrate by Way-of example only; the application of the principles of this invention, is that. made from silica and which, in the following paragraphs first generally discloses, and then later indetail, embjodi ments of our invention:

In themanner ofKistler, we first form a silica hydrogel. in a water medium.- The gel so formediswashed freeof solublesubstances left by the,in teraction.-,, We then dis.- place the aqueous menstruum by shakin'gpr extracting, the. gel repeatedly with a.solvent miscihle. in the, water, medium but-whichhas a critical temperature lowerthan, that of'water anda less solvent action than Water. These extracting liquids may be ofvarious kinds such as,the, lower alkylketones, lower. alkyl alcohols, lower alkyl fatty acids and their anhydrides. The ketone's, methyl, ethyljand propyl alcohols, for example, are entirely suit' able. The liquids, ofcourse must' reasonably dissolve the resins or resin forming materials which are added; as'will-be explained. In the main; acetone is preferrcd-f Tothe now water-free gel, itsmenst-ruum effectively-and substantially consisting of i one of the above solvents; are} added water-repelling agents which themselves-aresolnb le in the displacing-liquid'that-wasused: These: agents-we have. found may comprise substantially equivalent por -1 tionsof'polyhydric alcohols or-modified='alcoh'ols-(e.- g;- glycerol, glycol, glycerol-mon'ostearateandthelike) usedin conjunction with polycarboxylic acids (e. g. phth'alic;- suc'cinic, mali'c, citricand thelike); Further, additional alcoholsuseful .in this invention may comprise ethyleneglycol,- but'anediol, ethylidene-- glycol,- mesit-yle'nezglyco'l propylene-propanediol, pin acone; tetraphenyl-b'eniopina col, and sugarssuch as-thepehtoses;'hexoses-and glueosese also erythiritol and sorbitol. The equivalent anhydrides or condensation products of these bases and acids may also be used.

Experiment has shown'that a variety of other materials which are soluble in the last displacing solvent may also be used, all of which set up some degree of water-repellency on the Surfaces of the aerogel. Such substances are, e. g., aminosilanes, for instance, methyl aminosilane, butyl-aminosilane, polystyrene, siloxanes and derivatives of these and the like.

In addition, we have found that there exist a large number of other resinous forming compositions, for example, drying oils (such as linseed oil, tung oil, dehydrated castor oil and soya oil); natural resins (such as shellac, copal [manilla], damar, polyterpene, Canada balsam); synthetic resins (cellulosics, alkyds, ester gum, phenol formaldehyde, '[Epon] epichlorhydrin, bisphenol, coumarone indene); and cellulose derivatives (such as cellulose acetate, cellulose acetate-butyrate, ethyl cellulose, benzyl cellulose) which provide satisfactory water-repellent characteristics to the resulting aerogels.

We have found that the amount of water-repelling agents required to be added to the moist gel may vary over a wide range and yet produce varying degrees of eificient water-proofing, i. e. 10% to 100% by weight of the dry weight of the gel. The water-repelling agents of course are added always to the last solvent used in the displacing process. From this stage we again resume after the manner of Kistler by transferring the moist mass-now a composite of gel, solvent and water-repellent agent or constituent thereofto an autoclave wherein it is heated under pressure, as hereinbefore described, to a temperature. exceeding the critical temperature of the solvent or liquid phase used as a displacing medium. The vapour is then permitted to escape slowly to prevent disruption of the gel whereupon air or other gas is permitted entrance to the pressure vessel. The dried aerogel remaining is a product with essentially the same physical properties as those made known by Kistler with the added important characteristic of being substantially repellent to water.

.A detailed description of the method used to prepare the silica acetone gel is as follows:

To 200 gms. sodium silicate made up to 900 cc. with distilled water, 29 grams of 95% H2804 in 1100 cc. of distilled water were added. This produced a dispersion of silicic acid with a pH of 6.5. Other experiments showed that equivalent acids such as phosphoric, acetic, hydrochloric, etc. were equally satisfactory, and final pH values between and 9 could be used successfully.

After aging for 24 hours the gel formed above was agitated with 1 litre of distilled water until a smooth paste was formed, it was then filtered. This treatment was repeated to a total of four washings which substantially removed water soluble inorganic salts. Following this, the washed silica was suspended in one litre of acetone or alcohol and filtered. The silica now wet with a mixtureof water and acetone or alcohol was placed in a porous container which was in turn placed in a continuous liquid extractor and extracted with acetone or alcohol. Solid calcium chloride in the receiver, and a packed column placed in front of the condenser gave a continuous supply of substantially anhydrous liquid to the silica. The extraction was continued until all the water was removed from the silica and replaced with acetone. The silica was considered to be Water free when a few grams of acetonegel placed in 300 viscosity hydrocarbon oil showed no Sign of shrinking when the acetone was flashed off at 125 C. The continuous extraction normally lasted about 24 hours.

Various quantities of the silica alcogel or acetonegel which contained about 7.5% of silica by weight were made up to one litre with the appropriate liquid (alcohol or acetone). Into this dispersion was mixed the ingredients of, or the finishedrwaterproofing resin. (The exact amounts are shown in subsequent specific examples.)

The whole dispersion containing silica, solvent, and waterautoclave was closed and heating continued. The pres-- sure inside the vessel was not allowed to exceed 1600 p. s. i. This was controlled by opening the manually operated valve slightly. When a temperature about 1015 C. higher than the critical temperature of the liquid was reached the valve was released and the vapour passed through a condenser for collection for subsequent use. The autoclave was then placed in a cooling container. After reaching room temperature any excess of solvent vapour is pumped off and air admitted to the autoclave. A

The bulk density of the samples prepared by this method varied between 0.08 and 0.18 gm./cc. The

density depends chiefly on how thoroughly the water has been removed during solvent exchanging, and to a lesser extent on the concentrations of silica in the original gel,

and the thoroughness with which the solvent is removed after autoclaving.

For testing the water-repellency of the dried aerogels so produced the following method was used and followed with all examples illustrated hereafter:

10% by weight of the solid dried water-repellent aerogel was dispersed in hydrocarbon oil of 300 SUS at F. viscosity by passage through a colloid mill which provided a grease of good structure. A grease worker of 50 cc. capacity (6" in length, 1" in diameter, with piston having a single orifice instrument. About 5 gms. of grease with dried aerogel content was worked in each case with 40 cc. water at 70 C., the piston being operated manually in avertical direction forcing all the water and dispersed aerogel through the orifice of the piston twice for each complete cycle or stroke of the piston. For example, cycles of the piston resulted in 300 separate passages through the orifice. The degree of water-proofing of the aerogel is judged from the number of cycles of the piston required before the gel-oil dispersion disintegrates.

References in the following examples to strokes or.

cycles of the grease testing instrument indicate, therefore, the water-proofing, water-repellency, or waterresistance of the dried aerogel according to the invention and are a measure thereof.

ALCOGEL EXAMPLES Example 1 Silica alcogel 400 gms. containing 7.5% solids. Citric acid 8 gms. Glyceryl monoricinoleate 40 gms.

Autoclaved at 260 C. Yield 50 gms. of aerogel of density 0.12 gm./CC. Water resistancemore than 150 strokes.

Autoclaved at 250 C. Yield 40 gms. of aerogel of density 0.08 gin/cc. Water resistancemore than 150 strokes.

in diameter) Was the testing.

Water resistancemore than 150 strokes.

EXAMPLES OF ACETONE GELS (I. E. USING ACE- TONE AS SOLVENT) (The production of acetone gels was in fact preferred to that of the alcogels in order to reduce alcoholysis of the resin, although the procedure in each case was the same.)

Example Silica acetone gel 200 gms. containing 7.5 solids. Monostyrene 100 gms. Benzoyl peroxide 0.4 gm.

Autoclaved at 250 C. Yield 20 gms. of aerogel of density 0.06 gm./cc.

Water resistance--rnore than strokes.

Example 6 Silica acetone gel 400'gms. containing 7.5 solids. Citric acid 6.0 gms. Glyceryl monoricinoleate 30 gms.

Autoclaved at 250 C. Yield 50 gms. of aerogel of density 0.08 gm./cc. Water resistancemore than 150 strokes.

Example 7 Silica acetone gel 400 gms. containing 7.5% solids. Maleic anhydride 12 gms.

Glyceryl monooleate; 40 gms.

Autoclaved at 250 C. 7

Yield 55 gms. of aerogel of density 0.14 grrL/cc. Waterresistance-more than 150 strokes.

Example 8 Silica acetone gel 250 gms. containing 7.5 solids. Glyceryl monostearate gms. Citric acid 5 gms.

Autoclaved at 250 Yield 35 gms. of aerogel of density 0.18 gnL/cc. Water resistance-More than l50strokes.

Example 9 To 300 gms. of silica acetone gel containing 7 .5 solids was added gms. of a resin made by reacting maleic anhydride and 'glyceryl monoricinoleate in equimolecular proportions. This mix was autoclaved at 250 C. Water resistance--more than 15 0 strokes.

EXAMPLE OF ALUMINA AEROGEL (USING ACE- TONE AS SOLVENT) Example 10 Alumina Al(OH)3 was prepared by precipitating a solution of aluminium sulfate Al2(SO4)3.l8I-I2O with armmonia (NHiOH). The gelatinous precipitate was then Washed with water and solvent exchanged with acetone until the normal water content of the alumina was substantially replaced by acetone.

6 Alumina acetone gel 50 gin'srcontaining 12% solids.

Glyceryl v M monoricinoleate 265 gms. Citric acid 1.02 gms.

Autoclaved at 260 C. Yield 8 gms. of'aero'gel of density 0.15 gm./cc. Water resistancel0 strokes.

EXAMPLE OF CELLULOSE AEROGEL (USING ACETONE AS SOLVENT) Example 11 Cellulose was regenerated by acidifying viscose solution with dilute sulfuric acid. After Washing the cellulose with water it was solvent exchanged with acetone.

Aerogel was made as follows:

Gm's. Cellulose acetone paste 200 Citric acid 8 Glyceryl monoricinoleate 40 Autoclaved at 260 C.

The mixture was made up to 900 cc. with acetone and heated up to 260 C. The pressure was maintained at 1600 p. s. i. The acetone was released at 250 C.

The product was brown in colour and a solvent with a lower critical temperature than acetone would be more suitable.

The yield obtained was 37 grams with a bulk density of 0.249 grn./cc. It emulsified after 3 strokes were applied.

MISCELLANEOUS EXAMPLES Example 12 3 gms. dried silica aerogel not treated in the abovedescribed manner according to the invention (i. e., in the manner of Kistler). Water resistance-less than one stroke.

Example 13 Silica alcogel (equivalent to 3 gms. dried product) was treated in accordance with the method of the invention with 0.59 gm. phthalic anhydride 0.37 gm. glycerol 1.13 gms. oleic acid Water resistance12 strokes.

Example 14 Silica alcogel (equivalent to 3 gms. dried product) was treated in accordance with the method of the invention with 0.5 gm. resin derived from a composition resulting from a mixture of phthalic anhydride, glycerol and oleic acid in equimolecular proportions. Water resistance- 20 strokes.

Example 15 Silica alcogel (equivalent to 3 gms. dried product) was treated in accordance with the method of the invention With- 3.5 gms. glycerol monoricinoleate 1.2 gms. maleic acid Water resistance-over 50 strokes.

Example 16 As in Example 14, using acetone as final displacing solvent, but with 2 gms. resin formed from the ingredients set forth in Example 15. Water resistance-over strokes.

Example 17 As in Example 15, with 2 gms. of resin formed from the ingredients set forth. Water resistance'over 150 strokes.

7 Example 1 8 vention with- 2.6 gms. glycerol monoricinoleate 1.0 gm. citric acid Water resistance-over 150 strokes.

Example 19 Silica acetone gel (equivalent to 6 gms. dried product) was treated in accordance with the method of the invention with- 3.54 gms. glycerol monoricinoleate 2.02 gms. sebacic acid Water resistance-over 150 strokes.

Example 20 Water resistance-over 150 strokes.

It should be pointed out that in the above examples acetone was used as the final displacing solvent in which the resins or their producing agents were dissolved. It

is understood, of course, that any suitable solvent with low critical temperature may be used.

In addition to the liquids already mentioned which are useful in displacing water from the gels, other liquids such as ammonia, pyridine and lower mono-di and tertiary alkyl substituted amines have been found suitable.

With regard to the application of water-repellent characteristics to the silica gel when the agents used were various drying oils, natural resins, synthetic resins and cellulose derivatives, as hereinbefore mentioned, the procedure used was varied slightly for convenience. The silica acetone gel containing 4 grams of silica was mixed with an acetone or alcohol solution of each of these various materials in the amount as listed in the following table. The mixture was then placed in an autoclave and heated to 250 C. Small amounts of solvent were allowed to escape from time to time as the temperature was raised to keep the pressure just above the critical point without allowing large excess to build up. The pressure was held in the vicinity of 1400 p. s. i. After reaching the critical temperature, the solvent was allowed to escape completely. After the autoclave was cooled and evacuated, air was allowed to enter and the waterrepellent aerogel that was formed during the process was removed.

The efficiency of the coating was tested by dispersing 3 /2 grams of the dried gel in 26 /2 grams of 300 S. U. S. oil at 100 F., and 95 V. 1. Fifteen grams of this dispersion were placed in the cylindrical grease worker before described with 40 gms. of water at 70 C. and worked for 50 cycles of the piston. The water was replaced after each 50 cycles and the process continued up to 150 cycles or disintegration of the dispersion, whichever occurred first.

The following table indicates the interesting results obtained from this variety of agents. The coating materials used are set forth under four general classifications. Reference to strokes is a test of the waterrepellency characterictics of the coated gels in that it indicates the number of cycles of the piston required before the gel disintegrates (although, as mentioned, the testing was not proceeded with past 150 cycles of the piston).

For comparative purposes, 3 gms. dried silica not treated in the above-described manner according to the invention (i. e. in the manner of Kistler), was again tested 60 or organic.

and disintegration occurred with less than one stroke of the piston.

TABLE OF REsuI/rsf Amount of V V coating material Coating Material Classification used per Strokes 4 gms. silica (dispersed in silica 1O acetone gel) Gms. boiled linseed oil drying oil 85 tung oil (1 10 150 dehydrated castor 011.. 10 150 Soya bean oil .do 10 5 cellulose acetate-butyrcellulose derivative 10 150 ate. or synthetic resin. cellulose acetate -do 8 35 ethyl cellulose 8 104 benzyl cellulose 8 150 phenolic resin (Durez 10 150 10 150 10 53 i8 2 polyterpena gum damman- 10 150 gum copal (manilla) 10 80 Canada balsam 10 150 Congo resin 10 103 shellac 10 35 In the foregoing table of results Durez #12687 refers to a condensation product of a phenol and a formaldehyde and is sold under this trade name by Durez Plastics and Chemicals Incorporated, North Tonawonda, New York; Cumar V /2 refers to a polymer of coumarone and indene and is sold under this name by The Barrett Division Allied Chemical and Dye Corporation, New York.

Although 10 grams of resin were added in most of the above cases, this does not mean that the final product bears the ratio 10/4 coating material to silica. Most of these types of resins are appreciably volatile in the compressed solvent vapor and are carried over when the solvent is released.- Normally the carry-over would be recovered and reused and does not represent a loss. In other instances low solubility of the resins in the solvents used prevent complete solution and subsequent coating of the silica. In these latter cases the aerogel is probably a mechanical mixture of coated silica and coating material.

It should be pointed-out thatin the above examples acetone was used as the final displacing solvent in which the resins or their producing agents were dissolved. It is understood, of course, that any suitable solvent with low critical temperature may be used. For instance, they can include'gasolines, petroleum solvents, benzene and toluene. These latter are not, of course, soluble in water but could be used to replace another liquid in the gel if desired.

Also of interest is that finely divided .silica may be coated with, e. g. styrene, butadiene and butyl methacrylate to render its surfaces hydrophobic.

Our improved method of aerogel manufacture which in particular relates to the production of water-repellent aerogels is generally applicable to gels whether inorganic We wish to emphasize that the above examples are illustrative onlyand that silica hydrogel used so frequently in the above examples is used as a basis merely as a matter of convenience and for purposes of standardizing our testing methods.

It will be apparent that an improved process of aerogel production has been provided which culminates in the production of a water-repellent product that is much desired in the art and whose features are indeed beneficial and requisite in many articles of commerce. Only the preferred features of this invention have been'described 1. A process for making a polyester resin coated inorganic aerogel comprising replacing the water in a precipitated inorganic gel with a water miscible organic solvent having a critical temperature less than water, dissolving in the mixture so formed the reaction product of a polycarboxylic acid and polyhydric alcohol, heating said mass under pressure to a temperature in excess of the critical temperature of the organic solvent, allowing the gasified organic solvent to escape thus producing an inorganic aerogel having the surface thereof coated with a continuous film of resin.

2. A process for making a coated aerogel comprising replacing the water in a precipitated gel with a water miscible organic solvent having a critical temperature less than water, mixing in the dispersion so formed a water repellent film forming composition comprising one of the group consisting of a reaction product of a polycarboxylic acid and a polyhydric alcohol, natural resins, aminosilanes, natural drying oils, cellulose ethers and esters, phenol aldehyde resins, coumarone indene resin, heating the mass under pressure to a temperature in excess of the critical temperature of the organic solvent, allowing the gasified organic solvent to escape thus producing an aerogel having the surface thereof coated with a water repellent film.

3. A process as described in claim 2 wherein the polyhydric alcohol is one of the group consisting of glyceryl monostearate, glyceryl monooleatc and glyceryl monoricinoleate.

4. A process as described in claim 2 wherein the polycarboxylic acid is one of the group consisting of citric acid, tetrachlorophthalic acid, maleic acid, phthalic acid, hebacic acid and succinic acid.

5. A process as described in claim 2 wherein the solvent is one of the group consisting of lower alkyl alcohols and lower alkyl ketones.

6. A process as described in claim 2 wherein the film forming composition is a reaction product of a polycarboxylic acid and a polyhydric alcohol.

7. A process as described in claim 2 wherein the film forming composition is a natural resin.

8. A process as described in claim 2 wherein the film forming composition is a natural drying oil.

9. A process as described in claim 2 wherein the film forming composition is a cellulose ester.

10. A process as described in claim 2 wherein the film forming composition is a phenol aldehyde resin.

References Cited in the file of this patent Powell Jan. 13, 1953 

1. A PROCES FOR MAKING A POLYESTER RESIN COATED INORGANIC AEROGEL COMPRISING REPLACING THE WATER IN A PRECIPITATED INORGANIC GEL WITH A WATER MISCIBLE ORGANIC SOLVENT HAVING A CRITICAL TEMPERATURE LESS THAN WATER, DISSOLVING IN THE MIXTURE SO FORMED THE REACTION PRODUCT OF A POLYCARBOXYLIC ACID AND POLYHYDRIC ALCOHOL, HEATING SAID MASS UNDER PRESSURE TO A TEMPERATURE IN EXCESS OF THE CRITICAL TEMPERATURE OF THE ORGANIC SOLVENT, ALLOWING THE GASIFIED ORGANIC SOLVENT TO ESCAPE THUS PRODUCING AN INORGANIC AEROGEL HAVING THE SURFACE THEREOF COATED WITH A CONTINUOUS FILM OF RESIN. 